1. Field of the Invention
The present invention relates to a fixing device for fixing a toner image carried on a recording medium and an image forming apparatus including the same.
2. Description of the Background Art
A copier, facsimile apparatus, printer or similar image forming apparatus usually includes a fixing device for fixing a toner image carried on a paper sheet or similar recording medium. One of conventional fixing devices. includes a heat roller and a press roller facing each other. While the recording medium is conveyed via a nip between the heat roller and the press roller, the press roller fixes the toner image on the recording medium with heat.
Another conventional fixing device includes a fixing belt passed over a pair of rollers and substituted for the heat roller mentioned above. One of the pair of rollers faces the press roller also mentioned above. The roller, which drives the belt in cooperation with the other roller facing the press roller, accommodates a heat source for heating the inner surface of the belt. The press roller also accommodates a heat source for heating the outer surface of the belt. Generally, a belt is smaller in volume and thermal capacity than a roller and can therefore be heated more rapidly than a roller. In this sense, this type of fixing device implements a shorter warm-up time that the previous fixing device at the beginning of operation. In addition, the heat roller disposed in the press roller heats the belt for thereby further reducing the warm-up time.
When the rollers of the fixing device are formed of aluminum having high thermal conductivity, the belt may be provided with a two-layer structure made up of a base formed of stainless steel and a parting layer formed on the base and formed of silicone rubber or fluorocarbon resin, as known in the art.
The belt type of fixing device additionally includes a coating device for coating oil, or parting agent, on the belt. The coating device includes a coating roller or similar coating member held in contact with the belt for applying oil to the belt and oil feeding means implemented as a piece of felt impregnated with oil. The oil feeding means is held in contact with the coating roller over a period of time controlled in accordance with the duration of operation or that of non-operation of the oil feeding means, thereby feeding an adequate amount of oil to the belt. The coating roller is made up of a metallic core and sponge-like rubber covering the core.
Generally, an image forming apparatus is capable of dealing with not only plain paper sheets customary with, e.g., a copier, but also OHP (OverHead Projector) sheets, cards, postcards and other thick sheets of 90K or above and having a weight of 100 g/m2, and envelopes and other special sheets having greater thermal capacity that sheets. However, the problem with the belt type of fixing device is that oil coated on the belt is irregularly distributed on the belt in accordance with the cell diameter of the sponge-like rubber of the coating member. Particularly. when the recording medium is an OHP sheet, even fine irregularity of oil distribution on the belt directly translates into irregular transmittance. As a result, in a color image, a solid, yellow portion suffers from haze.
It is an object of the present invention to provide a fixing device capable of obviating fine irregularity of coating ascribable to the cell diameter of the sponge-like rubber of a coating member for thereby freeing an OHP sheet from short transmittance and haze, and an image forming apparatus including the same.
A fixing device for fixing a toner image carried on a recording medium of the present invention includes a heating member, a pressing member facing the heating member, a coating member for coating a parting agent on the heating member, and an agent feeding device intermittently brought into contact with the coating member for feeding the parting agent. Assume that a distance between a nip between the agent feeding means and the coating member, as measured in the direction of forward rotation of the coating member, is A, that a distance between a nip between the coating member and the heating member and a nip between the heating member and the pressing member, as measured in the direction of forward rotation of the heating member, is B, and that the circumferential length of the heating member is C. Then, a period of time T in which the recording medium enters the nip between the heating member and the pressing member satisfies a relation:
A+B+C/linear velocity of image formationxe2x89xa6T